1. Field of the Invention
The present invention relates to an irregular surface which is utilized on a bone implant to facilitate the growth of bone tissue within the surface. The invention also relates to a method of production of this surface. More specifically, the invention relates to the sequential etching of a bone implant surface to produce an irregular random pattern of protrusions and depressions through the use of chemical milling techniques.
2. Description of the Prior Art
In the field of bone implantation, or the use of man-made objects to replace portions of bone within the human body, there are two primary methods of affixing the implant device to the existing bone. The first of these methods involves the use of a cement or adhesive material which is applied to the surfaces of the implant and the bone. The cement is adapted to harden in a rapid fashion and rigidly affix the two portions in an immobile manner. The use of cement permits the application of loads to the joinder of the bone and the implant within a relatively short time following implantation. This is generally desirable in terms of the well-being of the patient, in that a quick physical recovery improves the overall recovery of the patient.
One of the significant shortcomings of a cement adhesion of the two elements is that over time, the cement tends to deteriorate. This deterioration may permit relative movement between the implant and the bone surface and if untreated, could allow the two joined elements to separate. In either event, the result is painful and dangerous to the patient.
A second method of affixation of the implant to the bone has also been utilized as an alternative to the cement technique. In this embodiment, the implant is provided with an irregular surface into which the bone may grow, creating a natural joinder between the bone and the implant. One of the shortcomings of this implantation technique, however, is the longer recovery time necessary to permit ingrowth of the bone into the surface of the implant. An additional problem which has occurred with relation to the ingrowth embodiment relates to the preparation of the surface of the implant. An implant having a smooth surface is inappropriate for use in this type of operation as it provides no gripping surface for the bone. An irregular surface, therefore, is preferred and in fact necessary for this application. Several methods have been proposed in the prior art for the preparation of the surface, such that a stable gripping surface will be provided into which the bone may grow.
Frye, U.S. Pat. No. 4,272,855, issued Jun. 16, 1981, discloses the use of generally conical projections emanating from the surface of the implant. These projections may be perpendicular to the surface of the implant or may be extending outwardly at an angle between 50.degree. and 90.degree., with respect to the surface of the implant. Frye teaches that an increase in the anchoring surface is a decisive feature which can influence and improve the bond between tissue and the implant. The projections described in Frye are generally regular in shape and devoid of corners and edges and have transition surfaces merging into the base level.
Van Kampen, U.S. Pat. No. 4,673,409, issued Jun. 16, 1987, discloses an implant having a surface comprising a multiplicity of spaced posts projecting from the implant surface for mating with bone material. The Van Kampen reference specifically teaches away from an edgeless surface as taught by the Frye reference. Van Kampen instructs that while a rounded surface minimizes the formation of stresses, it minimizes the total surface area that may be joined to the tissue, thus reducing the strength of the implant. Van Kampen discloses the use of regular posts which are roughly rectangular in cross-section. The posts are spaced at a regular interval and are formed by laser drilling.
It is evident from the teaching of these two references that there is some disagreement in the art regarding the best approach towards the preparation of an implant surface.
Another technique in the preparation of an implant surface is disclosed in Sump, U.S. Pat. No. 4,644,942, issued Feb. 24, 1987. The Sump reference discloses the use of a coating which is applied to the surface of the implant. The coating is comprised of a solid metallic powder and a solution of organic binders. A slurry formed of the two elements is applied to the surface of the implant and is permanently affixed thereto under controlled temperature and pressure conditions. The organic material is subsequently removed, leaving a porous, metallic coating on the surface of the implant.
Other techniques for applying a similar coating include plasma spray of a metallic material onto the surface of an implant resulting in a similar metallic irregular coating. While these porous coatings do provide an attachment surface into which bone may grow, these surfaces and the surface described in Noiles, U.S. Pat. No. 4,865,603, issued Sep. 13, 1989, exhibit significant shortcomings. The Noiles reference describes a surface in which furrows and depressions are cut or stamped into the surface of the implant. Each of these surfaces involves the addition of metallic material or the manipulation of the metallic surface of the implant. Each of these methodologies provides a surface that is subject to breakage and dislocation under stress. A metallic addition to the surface of the implant, even under rigorously controlled conditions, forms a joinder which is more easily broken than a singularly formed piece of metallic substrate. Similarly, the manipulation of the surface of the implant, even though formed of a single integral metal substrate, involves the stressing of the metallic surface which forms a locus for breakage when the implant is under a load.
What is lacking in the art, therefore, is an attachment surface utilized in conjunction with a metallic bone implant which has the structural integrity of a unitary element formed without the necessity of stressing that surface through manipulation. Additionally, what is needed in the art is a surface having the irregularities provided by an applied coating without the necessary lack of strength inherent in the joinder of the coating material to the implant substrate.